1. Field of the Invention
This invention relates to a diaphragm valve of the so-called direct seal structure used in a gas feeding system for a process of fabricating semiconductors or a process of producing liquid crystals and is operated so as to open or close a fluid flow path by causing a valve body formed of a diaphragm to make or break direct contact with a valve seat.
2. Description of the Prior Art
A diaphragm valve that is provided in the interior thereof with a seal structure formed of a metallic diaphragm and metallic seat packing has been prevailing to date. This diaphragm valve 1, as illustrated in FIG. 4, forms a seal structure with a so-called domed diaphragm 2 having a swelled central part and seat packing 3, disposes a vertically movable diaphragm-depressing member 4 above the diaphragm 2, and enables the diaphragm 2 to be actuated by vertically moving the diaphragm-depressing member 4 with a stem 5. At the time of closing the valve, by rotating a handle 6, thereby moving the stem 5 downwardly and depressing the diaphragm-depressing member 4 in the direction of the diaphragm 2 as illustrated on the left side of the cross section, a polished surface 2a of this diaphragm 2 is enabled to seal the seat packing 3 and consequently block a flow path 7. When opening the valve, by rotating the handle 6 in the opposite direction to the direction used in closing the valve, thereby moving the stem 5 upwardly and raising the diaphragm-depressing member 4 upwardly, as illustrated on the right side of the cross section, the diaphragm 2 is caused by its elastic force to restore the domed shape thereof and the flow path 7 is opened. The surface of the diaphragm-depressing member 4 for depressing the diaphragm 2 is formed with an arcuate surface 4a. By the contact of this arcuate surface 4a, the diaphragm 2 is enabled to maintain the state from the time of closing the valve till the time of opening it. By the depression exerted by this arcuate surface 4a on the diaphragm 2, the seat packing 3 is enabled to effect the sealing more positively and keep the flow path 7 in the closed state.
Then, a diaphragm valve forming a diaphragm in an upwardly spherical shape, causing this diaphragm to form on the circumference slightly to the center an annularly projecting part of an arcuate cross section having the upper surface dented downwardly and the lower surface projected downwardly, forming a valve seat destined to be pressed by the leading terminal of the annular projecting part of the diaphragm in an annular shape having a flat upper surface, and hardening the upper surface of the annular valve seat by a treatment is also known (refer, for example, to JP-B HEI 6-27551).
A diaphragm valve having a diaphragm sealed and immobilized by causing an annular projection formed along the peripheral part of the diaphragm to be nipped on the outer peripheral side and the inner peripheral side thereof with the annular projection formed for retaining a main body or the diaphragm and an annular groove formed on the opposite side from the diaphragm at the position corresponding to the annular projection is further known (refer, for example, to JP-A 2002-340203).
Meanwhile, a diaphragm valve of a structure other than the direct seal structure resorting to a diaphragm that has a disc 12 disposed in a lower part than a diaphragm 11 and a sealing member 13 inserted in the lower part of this disc 12 as illustrated in FIG. 5 is known. This diaphragm valve 10 is enabled, by vertically moving a stem 14 as illustrated in the cross section, to bring the sealing member 13 into contact with a valve seat 15 and open or close a flow path 16.
The diaphragm valve 1 of the direct seal structure shown in FIG. 4, however, has the possibility that the operation of opening the valve will entail the phenomenon of causing the diaphragm 2 to be drawn toward the flow path 7 and resulting in decreasing the flow volume because it is adapted to retain the diaphragm 2 solely by virtue of the state of causing the diaphragm depressing member 4 to be kept in contact therewith. Particularly, when the diaphragm valve 1 happens to have a large diameter, the surface area of the diaphragm 2 exposed to the pressure of the fluid is proportionally increased. When the value of this large diameter is used in the state of vacuum, the load of negative pressure possibly causes the diaphragm 2 to sink as though it were copiously drawn in the direction of the seat packing 3 and induces the valve to be spontaneously closed besides decreasing the flow volume.
Further, since in the diaphragm valve 1, the diaphragm 2 is adapted to form the domed shape by dint of its restoring force during the operation of opening the valve, it is required to be attached to the diaphragm-depressing member 4 in such a manner as to ensure formation of the domed shape. The diaphragm 2 is attached as adjusted for enabling exclusively the neighborhood of a top part 4b to contact the diaphragm-depressing member 4 during the operation of opening the valve.
When the diaphragm-depressing member 4 is moved downward through the motion of the stem 5, the diaphragm 2 carries out a stroking motion while it is deformed along the arcuate surface 4a from the top part 4b of the diaphragm-depressing member 4. At this time, the stem stroke L1 of the stem 5 in the vertical direction is converted to the valve stroke L2 which is the distance from the position which the top part 4b assumes when the valve begins to open to the position at which the seat packing 3 is sealed and the stroke (not shown) in the vertical direction of the amount of deformation produced by the diaphragm 2 along the arcuate surface 4a. Consequently, the valve stroke L2 of the diaphragm 2 to the seat packing 3 is shorter than the stem stroke L1 of the stem 5. When the stem stroke L1 of the stem 5 happens to be 1.8 mm in the diagram of FIG. 4, for example, the valve stroke L2 of the diaphragm 2 measures 1.0 mm. The valve stroke L2, therefore, is shorter than the stem stroke L1.
Thus, the diaphragm valve 1 has not been efficient because the valve stroke L2 acquired thereby is not equal in size to the stem stroke L1 and the flow volume acquired thereby is small for the size of the stem stroke L1. Further, the diaphragm valve 1 is required during the course of manufacture to have the stem stroke L1 set in advance at a prescribed magnitude for the purpose of obtaining the same magnitude as the valve stroke L2.
Further, an effort to increase the flow volume by forming the diaphragm valve 1 with a large diameter and enlarging the area for passing the fluid has resulted in suffering the amount of deformation of the diaphragm 2 relative to the arcuate surface 4a to increase further proportionately to the size of the area for passage in addition to entailing the aforementioned problem of the diaphragm 2 being drawn in the direction of the seat packing 3. As a result, the proportion of the valve stroke L2 to the stem stroke L1 is decreased and the proportion of the flow volume consequently obtained is decreased in accordance as the area for passing the fluid (seat diameter of the seat packing 3) is increased.
The diaphragm valve of JP-B HEI 6-27551 is aimed at increasing the Cv value which is the coefficient of flow volume by preventing the diaphragm from ruffling (furrowing) when the diaphragm is being sealed and by enlarging the stroke of the diaphragm, thereby widening the gap between the lower surface of the diaphragm and the upper surface of the annular valve seat during the operation of opening the valve. Since this diaphragm valve is not different from the countertype of FIG. 4 in possessing the structure of opening or closing the flow path by vertically moving the stem which is in the state of keeping contact with the upper side of the diaphragm in the domed shape, it cannot prevent the phenomenon of the diaphragm being drawn in the direction of the seat packing and cannot easily overcome the problem, such as the decrease of the flow volume.
As regards the diaphragm valve of JP-A 2002-340203, it is aimed at providing the diaphragm with a prescribed restoring force and consequently stabilizing the ability of the valve to open or close the flow path by having the inner and outer peripheral surfaces of the projection excluding the annular projection nipped and sealed through the diaphragm. The idea of causing the valve to assume an opened state by dint of the restoring force of the diaphragm, similarly to the invention of JP-B HEI 6-27551, falls short of preventing the decrease of the flow volume during the operation of opening the valve.
The diaphragm valve 10 which opens or closes the flow path by dint of the seal member 13 of the disc 12 as shown in FIG. 5 has the stroke of the stem 14 produce the valve stroke L3 of the disc 12 and is allowed to increase the flow volume during the operation of opening the valve because this valve stroke L3 can be increased. This structure of having the diaphragm 11 attached as nipped on the upper side of the disc 12 by helical attachment of the disc 12, however, induces the occurrence of a gap between the disc 12 and the diaphragm 11 and consequently entails the problem that the disc 12 and the diaphragm 11 will rub against each other and give rise to particles.
This invention has been initiated in view of the conventional problems and has for an object thereof the provision of a diaphragm valve of the direct seal structure capable of increasing the flow volume during the operation of opening the valve, namely a diaphragm valve that is capable of infallibly suppressing the generation of particles.